you've asked her to look after him so let her get on with it
I missed where the op said she'd asked her to look after him.
Personally I've been there and done that and would never have a regular family-childminding arrangement again.
My mum had mine at first, on her insistence. She doesn't work, she was really keen and wanted to have them. I also paid £100 a month...nowhere near childcare fees but just a token so that she wasn't out of pocket for any extras.
We were really grateful because it did save us a fortune. She did things differently to us at times but as young toddlers I just sucked it up when she ignored some requests.
As they got older (she had them for two years, from when they were 3 and 1 to 5 and 3) it only got worse. I'd make requests...please don't let ds2 have a nap after 5pm or he'll be up all night. Please don't let ds1 have free access to the biscuit barrel. Can you remember the steroid cream for ds2's bum (he had awful eczema around the nappy area). They'd be ignored. Ds1 would be stuffing ten biscuits every day after school. Ds2 was put down for a nap at 5.30 more and more often. She didn't put the cream on ds2 because in her opinion you didn't want to 'overload' then with meds and it was looking fine anyway.
The arrangement ended when we had a blazing row over food. Partly to make it easier for her and partly because I was very particular about food, I took their evening meal there every day she had them. Stuff like homemade stews or lasagne that could just be heated.
She started not giving them the meals I'd prepare. Over a few weeks, I'd turn up and be told 'oh I was cooking fish fingers anyway so they just had those' or 'ds1 said he didn't fancy lasagne so I did him some pizza and chips instead'. My mum isn't particularly great with healthy home cooking.
I politely asked so many times that she give the food I'd prepared...and after a while I put my foot down and said that that week three home cooked meals had gone in the bin and the dc had had fishfingers/pizza instead, it wasn't on and I wanted her to give them the food I made every day. It was a control thing and her response was basically that when they were under her roof, she'd do as she liked and I had no say on those days.
That was the last day she 'officially' minded them and me putting them with a childminder caused huge family ructions, two months of not speaking, tears about how cruel I was to dump them with a stranger rather than family.
Never again. Yanbu op. Just because it's family minding your dc doesn't mean you should completely relinquish all say in what happens on xyz day...because no matter who minds them YOU are the parent and others minding them should respect your choices or tell you that they can't do it.